The regions, with their names and numbering are taken from The Relative Hills of Britain by Alan Dawson.
The Corbetts
A Corbett is "a Scottish hill between 2500 and 2999 feet high with a drop of at least 500 feet (152.4m) on all sides". They are named after the list's original compiler, J.Rooke Corbett.
The Corbetts are a subset of the Marilyns, apart from Buidhe Bheinn, which is a twin top with Sgurr a'Bhac Chaolais on the Marilyn list but, oddly, appears separately on the Corbett list.
(Corbetts appear as Class=C on this website).
Corbett Tops
A Corbett Top is a Scottish hill between 2500 and 2999 feet high with a drop of at least 98 feet (30m) on all sides. Strictly the list of Corbett Tops should include the Corbetts, but it is more usual to see just the subsidiary tops listed as Corbett Tops (note that a Corbett Top can also be a subsidiary top to a Munro).
(Corbetts Tops appear as Class=CTM or CTC on this website, depending on whether they area a subsidiary top to a Munro or to a Corbett).
The Grahams
A Graham is "a Scottish hill between 2000 and 2499 feet high with a drop of at least 150 metres on all sides". They were originally referred to as Elsies, but have since been renamed after a compiler of a similar list, Fiona Graham.
The Grahams are also a subset of the Marilyns.
(Grahams appear as Class=G on this website).
Graham Tops
A Graham Top is a Scottish hill between 2000 and 2499 feet high with a drop of at least 98 feet (30m) on all sides. As with the Corbett Tops, usually the Graham Tops list just shows subsidiary tops of Munros, Corbetts and Grahams.
(Grahams Tops appear as Class=GTM, GTC or GTG on this website).